You won't catch John Durant in a tie. Shoes are optional, too. He has traded cubicle life for something a little wild: Promoting the diet and lifestyle of our ancestors from the paleolithic era. He's blogging and writing a book about his approach.

"For millions of years, we didn't have an obesity problem because we ate foods that our metabolism was adapted to," Durant says — foods such as root vegetables, tubers, fish and, of course, red meat.

"We were active and lived a healthy lifestyle," he says. Durant is one of many folks following the popular meat-laden paleo diet. He packs his freezer with deer meat and has found lots of places near his home in Manhattan to buy marrow bones and organ meats, as well as paleo-friendly barbecue joints for a meal out.

But modern medicine tells us that too much meat is bad for us, so what's a consumer to do?

During a workout at a CrossFit gym, a gathering spot for lots of paleo-enthusiasts, Durant told me it's no longer a challenge for him to avoid the onslaught of bagels and pizza at every street corner. The paleo approach is to eliminate grains and processed food, which are relatively new to the human diet. And, as a result, Durant says, he no longer gets the spikes and dips in his moods, and he feels better.

But Durant says it's a meat-based diet that was fundamental to early human development. (Check out our tongue-in-cheek Time Traveler's Cookbook: Meat Lover's Edition for more on this.)

My colleague Chris Joyce has reported on how a meat-based diet helped make us smarter.

And paleoanthropologist John Hawks at the University of Wisonsin, Madison, agrees: "We definitely evolved to eat meat."

"When we look at the fossils of early Homo [sapien], we see this immediate increase in the size of the body and also increase in the size of the brain," Hawks explains.

But that was then. Very few cavemen lived long enough to get heart disease or cancer. These are the reasons we're told to limit red meat consumption now.

"It really began in a big way in the Framingham study in the 1950s," says Michael Thun, an epidemiologist with the American Cancer Society.

"It [the study] found a relationship between total cholesterol and heart disease," Thun says. Over the years, there has been debate about whether high cholesterol is a cause or simply a marker of higher heart disease risks. But studies like this one helped raise the red flag about high-cholesterol foods, such as red meat.

Then, the evidence started mounting that people who ate daily servings of red meat increased their risks of developing certain cancers. For colon cancer, studies show that people who eat the most have about double the risk compared with people who eat the least red meat.

"That's been found in lots of studies," says Thun, "so it's pretty well-accepted."

Paleo enthusiast John Durant says he has thought about these studies and has heard the health experts, but he's not worried. He says lots of the people in these big epidemiological studies are sedentary and overweight.

He may be eating more red meat than the experts recommend, but he believes his paleo lifestyle, which includes running barefoot in Central Park, helps keep him thin, active and healthy. And he's not alone — the movement is attracting some medical professionals.

Because there are no studies of people who've been following the paleo diet, Thun says, it's hard to evaluate. "There's just not been enough people eating one kind of paleolithic diet to tell."

As for the rest of us who want to know how much red meat is too much, the best evidence suggests that cutting back to two to three servings a week is a good guide.

Copyright 2012 National Public Radio. To see more, visit http://www.npr.org/.

Transcript

RENEE MONTAGNE, HOST:

It's summer, and here at MORNING EDITION, we're ready to fire up the grill and talk meat.

STEVE INSKEEP, HOST:

During this time between Memorial Day and Labor Day, we're told Americans will eat some seven billion hot dogs. That number comes from the National Hot Dog and Sausage Council, of course.

MONTAGNE: And Americans will also wolf down millions of additional pounds worth of burgers, ribs, tenderloin, which got us wondering what it is about meat that makes us eat so much of it and how much is too much when it comes to our health. We asked NPR's Allison Aubrey to look into it.

ALLISON AUBREY, BYLINE: The crowded intersection at 17th and Broadway in Manhattan might not sound like the place to begin a story about meat, but it's here I met up with a guy named John Durant. He's on a mission to popularize the diet of our meat-eating ancestors.

JOHN DURANT: I've always had the potential to look like a caveman, but I think I've cultivated the look in the last few years.

AUBREY: With a scruffy dark beard that grows out in all directions, John Durant doesn't only look like a caveman, he's trying to live like one. At 29, he's given up his well-paid office gig to become a full-time promoter of the paleolithic lifestyle.

DURANT: For millions of years, we didn't have an obesity problem because we ate foods that our metabolism was adapted to. And we were active and lived a healthy lifestyle.

AUBREY: And that meant a diet of root vegetables, berries, tubers, fish and, of course, red meat. Now, simulating the paleo diet in modern-day Manhattan doesn't involve the challenge of hunting. In this urban jungle, where food is everywhere, John says the real challenge is learning to resist all the pizza and bakeries. We did not evolve to live in a bagel shop, he tells me. As I tag along with him, he shows me how he seeks out the kind of food he thinks made our ancestors strong and smart.

Are we looking to hunt, or are we looking to gather?

(LAUGHTER)

DURANT: We're opportunistic. We're going to do both. We're going to do both. Whichever good food sources we come across.

AUBREY: All righty.

As we turn the corner, he's drawn to the smoky aromas wafting from an open grill.

DURANT: Yeah, we got some barbecued chicken and beef and street meat here.

AUBREY: Is that calling your name?

DURANT: It's always calling my name.

AUBREY: I guess we don't have to hunt. The hunting's been done for us, right?

DURANT: The hunting's been done. The hunting's been done.

AUBREY: One advantage of modern city life. John says true to the caveman's diet, he is an omnivore, but he shies away from all grains and processed food, which he points out are relatively new additions to the human diet. Now, the menu at this barbecue place we've popped into turned out to be very paleo-friendly.

UNIDENTIFIED MAN: You came to the right place for meat. We also got a lot of great veggies over there.

DURANT: Yeah.

AUBREY: Sadly, there are no organ meats, like kidney or brains. But there are whole sides of pork and beef, and knives at the ready.

UNIDENTIFIED MAN: You like rare beef or well-done beef?

DURANT: Rare beef.

UNIDENTIFIED MAN: Rare beef? All right. Well, I got some really delicious prime rib, going to be juicy and bloody. Or I got beef shoulder. It's going to be a little more lean than the prime rib, but it's not going to be as rare. So...

DURANT: OK. Well, let's do the prime rib, juicy and bloody.

AUBREY: But there's much more to John's caveman lifestyle than just eating meat. To emulate our paleo ancestors, he sets himself some intense physical challenges. He runs barefoot in Central Park and pushes himself to the limit when he works out in a CrossFit gym. He's extremely wiry and fit, and he says eating this way no longer gets the spikes and dips in his blood sugar or moods that he had when he ate lots of bagels and carbs. He says he feels better.

DURANT: There is something primal about eating some ribs. And this meat's delicious.

AUBREY: Now, a diet rich in red meat is not what America's health experts recommend. But Durant's take: This is the diet that put our ancestors on the road to success.

DURANT: There's definitive evidence that for at least two million years, we have been using stone tools to butcher animals. And that was a way to get enormous number of calories and nutrients that allowed our brains to expand.

AUBREY: So, meat was essential to human evolution?

JOHN HAWKS: Absolutely. We definitely evolved to eat meat. I mean, that's very clear.

AUBREY: John Hawks is a paleoanthropologist at the University of Wisconsin-Madison.

HAWKS: And when we look at the fossils of early homo, we see this immediate increase in the size of the body and also increase in the size of the brain. At the same time, we have a real reduction in the size of the teeth. So it's clear that there's a diet shift that has happened, and that diet shift, giving us more access to energy, has enabled us to have larger brains.

AUBREY: OK. So that was then. But what about now? Cavemen usually didn't live long enough to get heart disease or cancer, and these are the reasons we are told to limit red meat. The red flags started to appear decades ago. Epidemiologist Michael Thun of the American Cancer Society says long-term studies of tens of thousands of people began linking our diets to our health problems.

MICHAEL THUN: It really began for cardiovascular disease in a big way with the Framingham study in the '50s that found this relationship between total cholesterol and heart disease.

AUBREY: And a major source of cholesterol is meat. Then the evidence started mounting that people who eat lots of red meat - meaning daily servings - also increased their risks of developing certain cancers. For colon cancer, it's about a doubling in risk.

THUN: That's been found in lots of studies, and so it's pretty well-accepted.

AUBREY: Now, our paleo enthusiast John Durant says he's thought about this, and he's not worried. He says lots of people in these big epidemiological studies are sedentary and overweight. He may be eating lots of meat, but he says his paleo lifestyle keeps him thin and healthy. And Thun says he wouldn't argue with this.

THUN: What I'm saying is that if you want to eat a paleolithic diet, that is far better than a Western diet, and I'm not going to argue with you about the meat consumption because the hugest thing is weight control.

AUBREY: Thun says the health risks of being obese are much more significant than the risks tied to individual components of a diet. He says in this day and age, there are so many different diet trends. Some people are going vegan. There's gluten-free, low-carb, high-protein, the paleo diet.

THUN: We tend to often argue around the edges of things rather than at core issues. But the big problem we have now is that, what, is it 36 percent of U.S. adults are now obese because our diet is just this blizzard of calories.

AUBREY: So, he says, as long as you keep your calories under control and eat plenty of fruits and vegetables, you should eat what you enjoy.

THUN: Reality is that you can eat no meat, or you can eat some meat. And depending on the kind of meat and what else you're doing, it will be fine for you.

AUBREY: As for that question how much is too much, for people who really enjoy red meat, experts say stick to eating two to three servings a week, with portions about the size of the palm of your hand. Even better: serve it up with a heaping plate of greens. Now, that's something even a caveman would enjoy. Allison Aubrey, NPR News.

(SOUNDBITE OF MUSIC)

MONTAGNE: We'll be keeping the grill hot and talking meat all this week. And if the paleo diet is making you think, you may enjoy what NPR's science desk has to say about the diet of our ancestors. Babylonian lamb stew, anyone? Check out "The Time Traveler's Cookbook" at npr.org. Transcript provided by NPR, Copyright National Public Radio.